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A nonlinear interior-point optimal power flow (OPF) method based on a current mismatch formulation in rectangular coordinates is proposed. In contrast to the classical way of solving the Newton equation that is involved, using a power mismatch formulation, the basic advantage of the proposed method is that, in the current mismatch formulation, some second derivatives become zero and some first derivatives become constant. In current mismatch formulation the elements Hij of the Hessian matrix are zero and the Jacobian elements Jii, Jij, Jji and Jjj are constant whereas in the power mismatch formulation for both cases those elements are functions of the voltage. These features make the computer code simpler. A theoretical comparison between the current mismatch formulation and the power mismatch formulation in rectangular coordinates is presented to point out their differences. Numerical examples on the IEEE 14-bus, IEEE 30-bus, IEEE 57-bus, IEEE 118-bus and IEEE 300-bus systems are presented to illustrate the nonlinear interior-point OPF method based on the current mismatch formulation. Preliminary results indicate that the two methods are comparable in terms of CPU time with the current mismatch formulation method having less computational complexity per iteration.